Field of the Invention
This invention is related to an optical synthesizer and in particular to one that outputs an electric signal based on a frequency difference between two light waves.
Description of the Related Art
Generation of high-frequency signals including those in the terahertz region has become indispensable in state of the art technical fields. A synthesizer as a measurement device generates signals of desired frequencies with high frequency resolutions. Electric signals (microwaves or millimeter waves) can be generated by a photomixer performing optical-electric conversion based on the difference between two coherent optical signals. Recently, the frequency bandwidth for photomixers has reached the terahertz region and photomixers are now more readily available.
Conventional optical synthesizers that perform phase synchronization control for a laser source by using a harmonic mixer are known. FIG. 3 shows an exemplary construction for a phase locked loop used for such an optical synthesizer. A highly stable laser from a reference laser source and a variable-wavelength laser are combined. The combined signal is distributed in two paths, one of which is sent to an output and the other to an optical-electric converter. After a frequency difference between the two light waves is converted into an electric signal (microwaves or millimeter waves), the harmonic mixer performs frequency conversion by using this electric signal and an external microwave signal, and a residual phase is used to perform phase synchronization control for the variable wavelength laser. An example of such a construction is disclosed in Cliche, J. and Shillue, B., “Precision timing control for radio astronomy, maintaining femtosecond synchronization in Atacama Large Millimeter Array”, IEEE control system magazine, pp. 19-26, 2006.
However, conventional optical synthesizers have a problem that the width of the frequency range of the signals that can be generated is limited.